As it is well known, ice crystals formed from a saline solution are composed of pure solids while the impurities remain in the solution or are separately precipitated. If salt water is frozen under conditions of violent agitation, large numbers of very small (10 to 250 micro-meter) ice crystals are formed and due to their natural buoyancy, they will collect at the top of the water. Such a mass of small crystals will tend to trap some of the salt water by capillary action between the crystals much like water soaked snow. If on the other hand, the salt water is frozen without agitation, the ice crystals grow to large solid blocks which inherently exclude all salts and foreign particles.
Previously proposed water purification processes employing a freezing cycle have experienced difficulties with the heat transfer surfaces. If an indirect heat transfer surface is located under water, it will soon be completely covered with an ice deposit which interferes with heat transfer from the cooling surface to the water and further creates a problem of harvesting of the ice. To avoid this difficulty, it has also been proposed to use a direct freezing process, for example, by bubbling a gaseous refrigerant such as Freon or butane or propane through salt water. Such refrigerants while expanding and evaporating, absorb heat from the water and produce water crystallation in the form of ice needles or platelets, which are suspended in the remaining brine and must be filtered and washed in order to harvest pure water. The difficulty with these processes is that all known gaseous refrigerants exhibit a certain degree of solubility in water and some carry-over with the water is therefore, unavoidable. Also, some refrigerants are flammable so that leaks present a fire hazard.